In recent years, as one of the methods for synthesizing liquid fuels from natural gas, the GTL (Gas To Liquids: liquid fuel synthesis) technique has been developed. In the GTL technique, natural gas is reformed to produce a synthesis gas containing carbon monoxide gas (CO) and hydrogen gas (H2) as main components, hydrocarbons are synthesized by the Fisher-Tropsch synthesis reaction (hereinafter referred to as “FT synthesis reaction”) with a catalyst using the synthesis gas as a source gas, and the hydrocarbons are hydrogenated and fractionated to produce liquid fuel products, such as naphtha (crude gasoline), kerosene, gas oil, and wax.
Generally, in the FT synthesis reaction, liquid hydrocarbons and gaseous hydrocarbons with various carbon numbers are synthesized as the hydrocarbons. Production of liquid fuel base stocks from the hydrocarbons synthesized by the FT synthesis reaction is performed by fractionally distilling the liquid hydrocarbons among the synthesized hydrocarbons, and by hydrocracking or hydrotreating each of the obtained fractions. When the liquid fuel base stocks are produced, the liquid hydrocarbons are fractionally distilled into a wax fraction (the boiling point exceeds about 360° C., and the carbon number is mainly C21 or more), a middle distillate equivalent to kerosene or gas oil (the boiling point is about 150 to 360° C., and the carbon number is mainly C11 to C20), and a naphtha fraction (the boiling point is lower than about 150° C., and the carbon number is mainly C5 to C10), depending on differences in boiling points (that is, differences in the carbon number).
Conventionally, as a hydrocarbon synthesis reaction apparatus which synthesizes hydrocarbons by the FT synthesis reaction, a configuration including a reactor which synthesizes hydrocarbons by the FT synthesis reaction by contacting a catalyst slurry having solid catalyst particles suspended in a liquid and a synthesis gas is known. In this reactor, as mentioned above, liquid hydrocarbons and gaseous hydrocarbons with various carbon numbers are synthesized as the hydrocarbons.
In the GTL process of producing liquid fuel base stocks from natural gas by the GTL technique, it is desired to improve the yield of the liquid fuel base stocks with regard to the natural gas which is a feedstock. Therefore, it is considered that, among the hydrocarbons synthesized in the reactor, not only the liquid hydrocarbons but also the gaseous hydrocarbons are used for production of the liquid fuel base stocks. Thus, as the hydrocarbon synthesis reaction apparatus, for example, as shown in the following PTL 1, there is known a configuration further including a flowing line through which the gaseous hydrocarbons flow to the downstream from the reactor, a cooling unit which cools the gaseous hydrocarbons which flow through the flowing line, and a separating unit which separates the liquid hydrocarbons condensed by the cooling unit from the gaseous hydrocarbons. In this configuration, the gaseous hydrocarbons synthesized in the reactor flow through the flowing line, and are cooled by the cooling unit, whereby some of the gaseous hydrocarbons are condensed into liquid hydrocarbons. Thereafter, in the separating unit, the condensed liquid hydrocarbons are separated from the lighter gaseous hydrocarbons. Thereby, it is possible to separate and recover some of the gaseous hydrocarbons synthesized in the reactor as liquid hydrocarbons, and it is possible to use the liquid hydrocarbons for production of liquid fuel base stocks.
As mentioned above, in the process of cooling and condensing the gaseous hydrocarbons by the cooling unit, gaseous hydrocarbons with a higher condensing point among the gaseous hydrocarbons are first condensed. Therefore, when classification is made on the basis of fractions of hydrocarbons in the production of the liquid fuel base stocks mentioned above, among the gaseous hydrocarbons, hydrocarbons of a wax fraction are first condensed, and hydrocarbons of a middle distillate are then condensed. In order to recover a large amount of liquid hydrocarbons from the gaseous hydrocarbons so that as many of the gaseous hydrocarbons can be used as possible for the production of the liquid fuel base stocks, it is desired to condense and separate not only the hydrocarbons of the wax fraction but also the hydrocarbons of the middle distillate.